Heart of Darknesstells the story of Marlow, a sailor, who describes to his shipmates the unusual experience he had traveling upriver in the Congo and the effect it had upon him. Hired by a Continental trading company as a steamboat captain between the outer stations and the interior, Marlow's primary mission was to visit and, if necessary, retrieve the mysterious Kurtz, an extraordinarily successful agent who had lost contact and reportedly fallen ill. Marlow tells the men that the entire journey was a sort of dream--lacking any real-world logic, deeply affecting, and difficult to describe in its details. The trip took several months, occurring in stages--a trip along the coast, an overland trek to the Central Station, and finally the riverboat journey to Kurtz's outpost.
During the entire expedition Marlow was struck by the mistreatment of natives by the Company and its agents, the preponderance of disease, the intimidating presence of the jungle, and the absurdness of the colonial operation carrying on for a relatively small amount of ivory. He began hearing of Kurtz as soon as he arrived, and everything he heard--of Kurtz's eloquence, of his high moral principles, of his effectiveness, of his influence in the Company--aroused Marlow's interest. The idea of Kurtz began to obsess Marlow. When they arrived at his station, they found he had set himself up as a sort of god to the natives he had once wanted to civilize; he had become more savage than even the natives, taking part in bizarre rites and using violence against the locals to inspire fear and obtain more ivory.
Against his wishes, Kurtz was taken back by Marlow and the other whites; his illness overcame him on the return trip, and he died. His last words--"The horror! The horror!"--were his realization of the depths to which he had sunk from his noble goals. He entrusted Marlow before his death with his papers, including an article he had written on bringing enlightenment and progress to the natives of the Congo. As evidence of Kurtz's decay, however, was the postscript he'd scribbled at the end of this article: "Exterminate all the brutes!". Marlow was shaken by his encounter with Kurtz, who had, because of his isolation, been exposed to the darkness within himself and had gone mad as a result. When back in Europe, Marlow contacted Kurtz's fiancé but could not reveal to her the terrifying last words.
Ultimately, Marlow tells the story of how when the thin shell of civilization has fallen away, the corruption and evil within can surface. Seeing the darkness lingering immediately under the surface of a man who thought himself moral forever affected Marlow as a deep nightmare would. As Marlow finishes his story, trailing off as he reaches the lie about Kurtz's last words, the sky has grown dark.
